The invention relates to a pulse-sound transducer in the ultrasonic range. Such transducers are necessary in various fields technology in which short pulses are necessary. As a first case there is defectoscopy which includes sonography in the field of medicine.
The classical construction of such a transducer comprises a plane parallel plate of piezoelectric material which has on the two broad upper and lower sides respective electrodes whereby the plate can be polarized perpendicularly to the sides which are covered with electrodes. This plate is cemented to a block which damps the ultrasonic waves and has an acoustic impedance which is matched to the piezo plate. On the output side so-called matching layers are provided which afford reflection-free sound transfer and with pulse operation can produce very short sound pulses. Transducers of this type belong to the known state of the art and a good discussion thereof and the problems arising therewith, for example can be found in the book of M. G. Silk, Ultrasonic Transducers for Nondestructive Testing, Adam Hilger 1984.
Transducers of the known type of construction require expensive technology and thus are costly where they are required to generate good pulses effectively. Furthermore, the known transducers are relatively thick (at least 5 mm) and it is thus practically impossible to fabricate them for frequencies greater than 30 MHz. In addition, with pulsed excitation, only relatively long pulses can be generated which have drawbacks for measurement purposes. A further disadvantage is that they are not suitable for automatic mass production and also in that their parameters cannot be maintained within a narrow tolerance range.
Relatively good pulse shapes and also good reproducibility require transducers with lens-shaped elemental blocks which, however, produce only weak signals. These transducers are significantly less sensitive in comparison to classical transducers. The same drawbacks have also been found for transducers which, because of special electrode configurations or inhomogeneous polarization of the piezo element, are capable of supplying relatively short signals.
The object of the present invention is to provide a sound transducer for the ultrasonic range which can emit strong and short pulses, has a high sensitivity and can guarantee reproducibility of the parameters in serial production.
This object is achieved in accordance with the invention with a pulse sound transducer for the ultrasonic range for use either as a transmitter or as a receiver with an elementary block composed of piezoelectric material. In accordance with the invention, the height of the elementary block composed of piezoelectric material of the transducer is greater than its width and the block at the output end for the pulse has a shoulder so formed thereon that a smooth output surface is formed for the sound wave. The block in longitudinal sections has a T-shape, whereby the base polarization runs perpendicularly to the output surface and the one electrode is provided on the output surface while the other runs above the shoulder on the block.
The block which is T-shaped in longitudinal section, can have a column shape, cone shape or pyramid shape with round, oval or polygonal cross section and is so dimensioned that a damping of the waves is effected which move within the interior of the column so as to prevent a reflection within the interior of the column at the free column wall and thus the emission of an after oscillation which can result in deterioration of the pulse quality. As a result additional damping means can be avoided. In addition the production of the transducer as a mass produced article is greatly facilitated by eliminating the additional damping means and the adhesive connection thereto. Essential for the invention is the formation of shoulders on the block to form the elementary cell. This shaping of the block and the selected proportions and the arrangement of the electrodes, which are disposed on the output surface and around the block above the shoulder, are decisive for the base oscillation which is thus of three dimensional configuration.
It is also important that, as a consequence of the construction of the elementary cell in accordance with the invention, that the electric field is closed within the elementary cell and thus such that a stronger pulse can be sent out. The base polarization direction of the piezo material should be perpendicular to the foot surface and thus the output surface for the pulses of the T-shaped elementary cell.
It has been found that it is especially advantageous for the following dimensional ratio to be maintained, namely, a/b/h=1/4-6/10, where a is the thickness of the shoulder, b the diameter of the block or its width and h is the height of the elementary cell. The size ratio of the sound generating element, here the elementary cell, is of special significance for all sound wave generating construction as examples from the music world show. Thus the violin, the viola, the cello and the contrabass generate different highs and lows of tonality based upon their different size proportions. It has also been found that an additional radial polarization by the application of a high voltage can improve the strength of the pulses. The highest probability is that this polarization utilizes the additional piezo effect advantageously.